As is well-known, the success of an optical fiber transmission system used in communications is related to the availability of solid state sources and detectors. Such detectors must be efficient, reliable and inexpensive and may be of a type APD (avalance photodiode), typically assembled in a hermetic package without a fiber leader. The optical fiberphotodetector assembly must posses high sturdiness and quality.
It is known that the package tightness is indispensable for guaranteeing protection from pollution produced by chip contamination agents that, owning to the high input voltages of approximately 200 V necessary for getting the avalanche gain, would reduce greatly the component life.
The connection efficiency between the optical fiber and APD through the glass window is ordinarily quite high, as the APD construction technique allows one to get good performances with a useful photosensitive area having a diameter comprised between 250 and 600 mm, to which corresponds a maximum distance of the fiber without connection losses comprised between 0.49 and 1.35 mm.
The classic solutions for the connection of the fiber with a detector supplied with a glass window and without a lead are numerous but not free from drawbacks. One conventional and frequently used device (used by the applicant itself) is shown in schematic section view in the FIG. 1. It is formed of a substantially cylindrical bush (BU); its lower side and hole (PA) hold the APD that terminates on its upper side with the plate of the glass window (VE). The monofiber cable (CA) with insulation (G) penetrates for a substantial depth (H) into the upper cylindrical part of the bush (BU) and continues into the lower zone without insulation as an uncovered fiber (FO) to a depth (h).
Generally, positioned between the upper cylindrical part (BU') of the bush (BU) and the passage (PA) having a smaller diameter, is a truncated conical fitting (BU"). As the drawing shows, the fiber end (FO) is kept at a very small distance from the glass (VE). The cable (A) and its appendix (FO) are secured in the position shown by filling all the inner bush volume (BU) with resin (preferably opaque in the upper part and transparent in the lower part).
The alignment of the termination end of the fiber (FO) with the APD was typically made by means of micromovements of the fiber. Among the more important drawbacks of this type of assembly are the following:
the jacket (I) between the free lower fiber end (FO) and the lower surface of the glass (VE) holds a resin stratum that, in some conditions, damages and breaks the glass (VE); PA1 the jacket (I) height between (VE) and (FO) is notoriously critical as the maximum coupling is obtained only at a precise position.
The adjustment of (I) is not easy in the illustrated prior art arrangement. If the free fiber end is pushed too much against (VE), the fiber is bent until its breakage; on the contrary, if it is positioned far from (VE), coupling efficiency is lost.